Fibers made from melamine resin are dyeable under conventional disperse dyeing conditions without significantly degrading the fiber properties. In this regard, the melamine fiber content of the melamine fiber-containing articles is not significantly reduced when they are dyed in pH ranges from 4 to 10, unless very high temperatures or prolonged dyeing times are employed.
However, under conventional disperse dyeing conditions it is very difficult to achieve deep shades on articles containing melamine fibers, even with the inclusion of producer-colored companion fibers such as pigmented meta- or para-aramids. Disperse-dyed blend fabrics also show inferior wash and/or light fastness. While deep shades can be achieved using acid dyes at a pH below 4.0, under such conditions, there can be loss of an unacceptable amount of melamine fibers due to acid hydrolysis, particularly at elevated temperatures. Deep shades can also be achieved using acid dyes for the melamine fibers and basic dyes for the aramid fibers. These fabrics formed of a blend of melamine and aramid fibers also show poor crock and wash fastness in deep shades.
Recently, it has been proposed to dye condensation products obtainable by condensation of a mixture comprised of unsubstituted melamine, substituted melamine and hydroxyphenyl compounds with formaldehyde or formaldehyde donor compounds in the form of fibers, yarns threads, wovens, knits or nonwovens, in an aqueous liquor with one or more dyes of the class of azo, anthraquinone, coumarin, methine, azamethine, quinophthalone or nitro dyes. (See U.S. Pat. No. 5,624,466 to Schindler et al, the entire content of which being expressly incorporated hereinto by reference.) Also, it has been proposed to dye articles composed of a blend of melamine and cellulose fibers by the use of dyes which preferentially dye the cellulose fibers and not the melamine fibers so that when such a fiber blend is incorporated into a fabric, a chambray appearance results. (See U.S. Pat. No. 5,830,574 to Gadoury, the entire content of which is expressly incorporated hereinto by reference.)
According to the present invention, articles formed of melamine fibers can be dyed to deep shades with minimal (if any) loss of the melamine fibers due to hydrolysis. More specifically, according to the present invention, melamine fiber-containing articles are dyed to deep shades using a dye bath containing an acid dye in combination with an acid donor. During the dyeing process, the dye bath is slowly lowered to provide maximum exhaustion of the dyes at the higher temperatures employed, while limiting the amount of time the melamine fibers are exposed to the lower pH and higher temperature conditions.
These aspects, and others, will become more clear from the following more detailed description of the preferred exemplary embodiments thereof which follow.